The present invention relates generally to three-dimensional forms having high aspect ratio features with ultrasmooth surfaces, and to methods for fabricating such forms useful as microdevices. More particularly, the invention relates to such microdevices having an interior cavity with surface features on the order of one micrometer, yet retaining sub-micrometer surface roughness, and to a method for fabricating such microdevices using a precise contour polishing method.
Microdevices are microelectromechanical systems (MEMS) with overall dimensions on the order of one millimeter and feature sizes on the order of one micrometer. One approach to fabricate such MEMS devices employs LIGA technology (LIGA is an acronym from the German words for lithography, electroplating, and molding). In particular, the LIGA technology can produce precise, high aspect ratio microstructures from engineering materials.
Designs for microdevices call for such things as precision cavities having uniaxial symmetry with micrometer-size features on the interior surface. These microdevices include resonance cavities and bellows. Additional applications include microscopic battlefield recognizance robots, smaller or microscopic spacecrafts, portable medical systems, communication equipment, microfluidic or microanalysis systems, or other microsystem applications.
Some of the techniques currently used for the manufacture of such microdevices involve electric discharge machining (EDM), fusion bonded LIGA parts, and diamond machining. However, these alternative technologies do not offer satisfactory solutions for the microdevice applications described above. EDM and diamond machining yield rough surfaces and LIGA parts limit the geometries available. High aspect ratio processing via EDM, LIGA, diamond machining and other micromachining technologies are generally described in Chapter 6 of the Handbook of Microlithography, Micromachining, and Microfabrication (P. Rai Choudbury, ed., SPIE-The International Society for Optical Engineering and The Institute of Electrical Engineers, 1997) and Chapter 7 of Fundamentals of Microfabrication (Marc Madou, CRC Press, 1997).
The microdevices claimed in this invention have micrometer-size interior surface features and sub-micrometer RMS surface roughness. The method claimed is cheaper, faster, and produces much better devices than alternative micromachining technologies. The inventive method uses photolithography to produce a shaped bit, contour polishing to produce a contoured sacrificial mandrel, subsequent coating of the mandrel with a structural material, and removal of the mandrel to produce the microdevice. Contour polishing is to be distinguished from microcutting techniques, which are not generally capable of producing such fine features and surface smoothness.
U.S. Pat. No 5,236,572 discloses a method for forming inkjet orifice plates comprising electroforming a metal layer on the surface of a reusable mandrel, such as a movable belt or rotating drum, having a microfabrication pattern and subsequently separating the metal layer from the mandrel surface. The continuous electroforming process of this patent is capable of forming a metal surface with details that a micrometer-sized in dimension, but not on the interior surface of a microdevice.
U.S. Pat. No. 5,772,903 discloses a process for making tapered capillary optics comprising producing an etched metal or glass wire mandrel having extremely low surface roughness, coating the wire mandrel with a chosen material, and removing the wire mandrel by chemical etching. Such a process can produce a microdevice having an interior cavity with an ultrasmooth bore, but it is not capable of producing precise contoured features on surface of the bore.
The present invention relates generally to three-dimensional forms having high aspect ratio features with ultrasmooth surfaces, and to methods for fabricating such forms useful as microdevices. More particularly, the invention relates to microdevices having an interior cavity with surface features on the order of one micrometer, yet retaining sub-micrometer surface roughness, and to a method for fabricating such microdevices using a precise contour polishing method.
To produce such a microdevice requires the preparation of a sacrificial mandrel that has the precise surface contour to be replicated in the microdevice itself. This mandrel must have a replicating surface that is substantially free of roughness, on a scale of about one micrometer RMS or less, and has the desired surface contour. A convenient mandrel material is a plastic, such as polymethylmethacrylate (PMMA), that can be easily polished and readily removed from the structural material that forms the microdevice.
To produce a contoured surface having precise high aspect ratio features requires a shaped bit having features of similar dimension. A shaped bit allows for the generation of complex geometries in one operation as opposed to traditional cutting where the cutting tool must make serial cuts. Cutting tools also require higher maintenance - a sharpened cutting edge must be whet and kept from dulling. In one embodiment of this invention, a method for making such a shaped bit is the LIGA process, which has been demonstrated to be capable of producing precise, high aspect ratio microstructures of suitable bit materials. LIGA bits are used because their thickness keeps them from being elastically or plastically deformed.
The sacrificial mandrel is contoured by slowly advancing the shaped bit into the rotating sacrificial mandrel that is mounted in a precision turning machine, such as a precision lathe, and whose surface has been prepared with a suitable polishing compound. The contouring is preferably accomplished by polishing but may also be accomplished by cutting under limited circumstances. However, cutting tends to leave an undesirably rough surface.
The structural material can be applied to the mandrel by electroplating or by other methods known to those of skill in the art, such as vapor deposition or plasma spraying.
Where the mandrel material is nonconductive and electroplating produces the structural material, it is generally necessary to first apply an electrically conductive precursor coating to the surface of the mandrel. If the structural material is applied to a thickness in excess of the final thickness, the external surface can be machined to its final shape. The mandrel is subsequently removed by degradation, dissolution, leaching, vaporization, or other removal process that does not attack the structural material itself.